It is necessary in many industrial processes to be able to monitor temperature, pressure, volume and/or speed of a fluid flowing in a pipe at a multiplicity of locations. In some situations the sensors are connected permanently to a central control room, and in others the probes of the sensors remain in place at the monitoring locations, but are only connected periodically to some device capable of reading them, as when there has been a problem and it is being traced. To this end the meter is provided with a jack into which a plug of a portable reader can be inserted to make the required readings.
Such a meter is normally mounted as a fitting right in the line having the flow to be monitored. The meter therefore has a housing and is formed with a main passage that constitutes a continuation of the conduit sections to both sides of it, and also has a lateral branch passage by means of which the appropriate sensor is mounted for exposure to the fluid in the main passage.
Flow or volume measurement particularly requires the provision right in the stream of a probe element which is deflected by the flow. Such a device might be a Woltmann-type fan which is rotated by the flow, or a bendable element whose magnetic field is detected. These devices therefore obviously create a service problem, as they are exposed to foreign matter, the caustic or acidic phase of the fluid, and so on.
Thus it is necessary to service these devices at least occasionally. Such work requires that the line they are in be shut down and, usually, drained. Once the servicing is complete, it is necessary to bleed the line in most cases also. As a result it is standard practice to replace all of the delicate sensor elements at one time to avoid having to drain the line too often, even though replacement of the works of such a meter is usually a relatively simple and rapid task.
Furthermore, such flowmeters are normally combined with other sensors, such as temperature meters and pressure meters. In case of failure of any of these parts it is therefore also necessary to shut down and drain the line. It has been suggested to avoid this problem by providing such devices in a separate branch line that can be isolated from the main line during servicing, but such a system cannot work for a flow meter which must be right in the main line, and merely adds to the overall cost and complexity of the monitoring device.